Ammonia or urea may be stored on-board a vehicle for use as a reductant in exhaust emission control systems.
Under some conditions, such as during ammonia or urea recharge, ammonia vapor that was generated in the reductant system may be used, such as described in U.S. Patent Publication No. 2006/0117741. For example, ammonia vapors may be routed to a storage unit and a reactor that uses catalytically generated heat from the exhaust or electrically generated heat. Further, the vapors may also be directed to the exhaust gas to react with exhaust gas NOx in a catalyst in the exhaust flow.
However, the inventors herein have recognized several issues with such a system. For example, ammonia vapors may be used inefficiently in the exhaust, such as during various SCR (selective catalyst reduction) catalyst temperature conditions.
Thus, in one approach, a method of managing vapors generated from an ammonia-containing reductant delivery system for an engine of a vehicle may be used. The method comprises storing ammonia containing vapors generated in the reductant delivery system and purging said stored ammonia into selected regions of the engine based on an operating temperature, such as temperatures of the regions.
Thus, the reductant may be directed to various regions of the engine based on operating conditions. For example, at relatively high operation temperatures, ammonia may reduce a first amount of NOx in the combustion chamber and then reduce residual NOx in a downstream SCR catalyst. In another example, at low operation temperatures, ammonia may be used to recharge a urea or ammonia storage device to be later released into the system. In this way, it may be possible to reduce vapor pressures and reduce un-metered ammonia exiting the system. Further, by later use of the stored ammonia in the exhaust, more effective usage rates of ammonia (or urea) may be achieved, possibly reducing the amount or frequency of ammonia (or urea) that is added by a user.
Further, in an example approach where stored ammonia or urea is purged into the engine combustion chamber, it is possible to reduce some amount of NOx prior to a downstream SCR catalyst. In this way, the combination of the combustion chamber and the SCR catalyst may increase the total amount of NOx reduced in the system. As such, the engine may run on leaner conditions to improve fuel efficiency.
Finally, in still another example approach where ammonia or urea is stored during a reductant filling and later purged (into engine, exhaust, or back to tank), it is possible to reduce evaporative emissions that may be released into the atmosphere during reductant refilling. As such, the operator may be subject to a reduced amount of bothersome ammonia vapors.